Vapor discharge lamps, such as a rubidium lamp, are used as energy sources having a defined spectral content for optical pumping and atomic absorption in atomic frequency standards. Such vapor discharge lamps are generally excited by an application of radio frequency energy from an electronic power oscillator. Such vapor discharge lamps require an electronic power oscillator, or excitor, that can reliably start the lamp and maintain constant lamp output under varying conditions. Temperature and/or component variations in the oscillator circuit can change the lamp output, both in intensity and spectral distribution. In addition, some variations in the oscillator power supply, such as low frequency ripple, can impress disturbances on the light output. Likewise, variations in the load presented to the oscillator by the vapor discharge lamp can induce variations in the excitation power and thereby cause periodic fluctuations in the lamp output. Variations in lamp output due to temperature changes, component variations, and variations of the excitation power are typical of the difficulties often encountered in starting and sustaining an electrodeless vapor discharge lamp using conventional excitation circuity.
Typically, a vapor discharge lamp, such as a rubidium lamp, is ignited by an r.f. excitation field generated by a coil driven by an r.f. oscillator. Unfortunately, the impedance of the lamp traverses extremes, equivalent to a pure capacitance prior to ignition and, after ignition, becoming a complex impedance changing with the level of the r.f. ignition signal and the lamp temperature.
In addition, lamp oscillator circuits continue to be plagued by a phenomenon known as "blocking oscillations", in which the lamp oscillator generates a low frequency amplitude modulation of the r.f. excitation output of the lamp oscillator at frequencies such as for example, around about 150 kHz. Since the light output of the lamp oscillator is a function of the r.f. power to the bulb, such blocking oscillations will modulate the light intensity out of the lamp system at the blocking oscillation frequency. The phenomenon of blocking oscillations is not fully understood and there presently is no analytical prediction of the frequency or percent modulation of the blocking oscillations. The blocking oscillations, however, tend to appear at lower oscillator transistor current levels in field effect oscillator transistor (FET) oscillators than in bi-polar transistor oscillators.
In an FET oscillator circuit, for example, a portion of the output r.f. excitation signal is fed back to the input of the oscillator FET to provide the desired regenerative feedback for purposes of sustaining the r.f. output. The current through the transistor is proportional to the square of the feedback voltage, with a drain to source voltage (VDS) being greater than the threshold voltage (VTH) of the FET and for voltages from gate to source (VGS) greater than VTH. Accordingly, with a symmetrical r.f. feedback signal about the DC bias level on the gate of the FET, the positive excursions of the r.f. feedback signal increase the transistor current more than the negative excursions decrease the transistor current. As a result, an increase in the r.f. amplitude of the feedback signal will increase the DC current through the FET, which then increases the oscillator loop gain, which in turn encourages the increased r.f. amplitude, thereby establishing an undesirable regenerative feedback loop resulting in the blocking oscillations. Other factors which may contribute to the blocking oscillations include the RC time constant of the r.f. feedback path and the oscillator's output r.f. amplitude response time to DC current changes.
Although the blocking oscillation generation has been described above with respect to the operation of an FET transistor implementation, the same problem exists in a bi-polar transistor implementations as a result of the logarithmic Vbe to Ic relationship. In all lamp oscillator circuits, however, it is desirable to not have the blocking oscillations occur at transistor currents near the intended operating range.
A number of attempts have been made to stabilize the output the r.f. oscillator used to drive the vapor discharge lamp. For example, U.S. Pat. No. 4,456,891 to Fowks discloses an r.f. oscillator circuit for igniting a rubidium lamp in a rubidium frequency standard which is adjustable in power and regulated with a fixed DC voltage and which utilizes current sensed feedback to permit r.f. oscillator power variations to compensate for variations in impedance. Furthermore, when the rubidium standard is initially turned on, the lamp heater is monitored and the lamp r.f. power is forced to maximum until the proper heater temperature is reached to reduce rubidium lamp warm-up time and assure proper lamp ignition. After the appropriate heater temperature is reached, the lamp r.f. power is automatically reduced to a preset value which may be determined by a manual setting of a potentiometer in the current sensed feedback circuit of the oscillator. In addition, a light sensing feedback circuit employing the photodiode of the rubidium frequency-generating cell is utilized to apply a delayed level sensitive control to the r.f. oscillator to maintain proper lamp ignition.
U.S. Pat. No. 4,721,890 discloses a system in which the output of an alkaloid vapor lamp for use in an optical pumping system is stabilized by use of a feedback circuit which regulates current flow of an electronic power oscillator used to excite the alkaloid vapor lamp. Starting of the alkaloid vapor lamp is facilitated by increasing the supply current to the oscillator until the alkaloid vapor lamp is lit.
In spite of the prior efforts to stabilize the operation of the oscillator circuitry for use for the ignition and sustaining operation of a vapor discharge lamp, improvements in the means for exciting the vapor discharge lamps of atomic frequency standards are needed, including improvements in lamp oscillator circuits.